All multicellular organisms have evolved mechanisms to perceive and respond to extracellular chemical signals. Among them, peptides are the most common mediators of intercellular interactions in animals because they provide great variety in their sequences, lengths and/or post-translational modifications (PTMs) to represent different physiological responses (Boller, 2005). In contrast to peptide discovery in animals, only a few signaling peptides have been identified in plants (Farrokhi et al., 2008a; Butenko et al., 2009). It is expected that most of the endogenous plant signaling peptides that play prominent roles in intercellular communication are still undiscovered. Because the complete sequencing of the Arabidopsis genome has revealed that plants have up to ten times as many predicted peptide receptors (Shiu and Bleecker, 2003) and transporters (Initiative, 2000) as animals. Moreover, among the currently identified peptides in plants, only relatively few have been found to function in defense signaling. This is mainly due to the fact that defense signaling peptides are mostly derived from the selective action of proteases on larger precursor proteins, are expressed at low levels, and are highly dynamic.
It is already known that tomato wounding can induce an anti-herbivore response, which is regulated by the peptide hormone systemin, and the small molecule hormone jasmonic acid (JA) and its methyl ester, MeJA (Pearce et al., 1991; Orozco-Cardenas et al., 2001). Systemin was the first identified signaling peptide and also the first confirmed peptide elicitor of damage associated molecular patterns (DAMPs) in plant. It is expected that several signaling peptides are involved in combating herbivore and pathogen attack (Cheong et al., 2002; Francia et al., 2007; Chassot et al., 2008), but the details of the regulation of anti-herbivore and anti-pathogen responses by peptides during wounding stress still await elucidation. Several DAMP peptides have been discovered in other plant species and suggested to be bioactive in tomato (Boller and Felix, 2009b; Campos et al., 2014); these include HypSys (Pearce et al., 2001a; Narvaez-Vasquez et al., 2007), RALF (Pearce et al., 2001b) and Pep1 (Huffaker et al., 2006; A. P. Trivilin, 2014). Pep1 was clearly identified to be pathogen-related in Arabidopsis and its putative precursor in tomato was recently found to involve in the anti-pathogen response (A. P. Trivilin, 2014). However, its endogenous level in tomato has not yet been proved to be induced by tissue damage or MeJA, a potent inducer of systemic wound signaling and response in tomato (Scheer and Ryan, 1999). To our knowledge, no study to date has quantitatively profiled the global change in cellular peptide expression in plants before and/or after the induction of stress responses.
There is a need to identify new defense signaling peptides in plants which not only to advance plant stress biology, but also to aid in the development of alternative ways to improve stress tolerance or resistance for better crop productivity and minimization of the use of agrochemicals (Pearce et al., 1991; Pearce et al., 2001a; Huffaker et al., 2006).